Adjustable precision potentiometers



Sept 17, 1957 5. 0'0. WOODS ETAL 2,806,928

ADJUSTABLE PRECISION POTENTIbI AETERS Filed on. 15, 1954 4 Sheets-Sheet1 INVENTORS a Q Whalep 1957 v IHI. 0'0. WOODS El'AL 2,806,928

ADJUSTABLE PRECISION POTENTIOMETERS Filed Oct. 13, 1954 4 Sheets-Sheet 2INVENTORS a. m

Sept 17, 1957 E. OD. wooos ETAL 2,806,928

ADJUSTABLE PRECISION POTENTIOMETERS Filed Oct. 13, 1954 4 Sheets-Sheet 3IN V EN TORS Sept 17, 1957 a 0'0. woops ETAL 2,806,928

ADJUSTABLE PRECISION POTENTIOIIETERS Filed Oct. 13,- 1954 4 Sheets-Sheet4 7 J J 0' 1' 0% V I mz V G5 7/ 25. 8 N 7 G mo may:

United States Patent ADJUSTABLE PRECISION POTENTIOMETERS Elmer ODellWoods, Santa Monica, Wilbur John Mac- Cauley, Northridge, and NeilAnthony Kaufman, Venice, Calif., assignors to Helco ProductsCorporation, Santa Monica, Calif.

Application October 13, 1954, Serial No. 461,986 6 Claims. (Cl. 201-56)This invention relates to variable resistance devices such aspotentiometers, and more particularly to improved adjustable precisionotentiometers. The term potentiometer as used hereinafter will beunderstood as pertaining to variable resistance devices in general.

The requirements of precision potentiometers such as those of a rotarytype variable resistance device or potentiometer used in variousinstruments, for example in control devices for aircraft and guidedmissiles, are very precise, forcing the technician to work with verysmall dimensional tolerances in all phases of their manufacture. Thelinearity, resolution, angle of rotation, total resistance, terminalresistances, and the electrical centers must in all cases be veryprecise and within very small tolerances. It can readily be seen thatwithout adequate provision for adjustment and keeping a preciserelationship between the above mentioned variables and the othervariables that enter intosthe art of making precision potentiometers,the manufacturing costs may become excessive and prohibitive for manyapplications.

While the invention is broadly applicable to control devices, it hasspecial utility in a multiple or ganged potentiometer of the rotary typefor use in an instrument data computer system. i

The general object of the invention is to provide a rugged, efiicient,highly accurate potentiometer of relatively simple construction that maybe manufactured in quantity with relatively low labor costforfabrication and assembly.

More particularly, an object of this invention is to provide means foradjustment of the total resistance of the potentiometer readily andmanually.

Another object is to provide a potentiometer compensating device adaptedto compensate a potentiometer to have a precise predetermined totalresistance.

Another object of this invention is to provide a potentiometercompensating device adapted to compensate a potentiometer to provide apredetermined resistance vs. a mechanical function.

Another object is to provide an etticient, trouble-free arrangement forshorting out a predetermined portion of a potentiometer resistor so thata-constant voltage is maintained over the corresponding portion of thepotentiometer action.

Another object is to provide a means for compensating for variations inmechanical movement so that the electrical percent of potential may becorrected to equal the percent of mechanical movement.

Another object is to provide a means for accurate location of the zeropercent and 100 percent of electrical potential of the potentiometer inrelation to the mechanical length.

Another object is to provide means for accurate division of potentials.

Another object is to provide a means by which the desired voltagefunctions can be set with extreme precision from data presenting thefunction in a tabular form,

2,806,928 Patented Sept. 17, 1957 by shifting taps along the resistor orby measuring the amount of resistance between successive taps.

Still another important object is to provide an improved arrangementwhereby the adjustment or synchronization of a plurality ofprecisiontype variable resistors or potentiometers may be obtained in arelatively simple manner.

Another object is the provision of means for accurately matching oraligning electrical centers, of ganged or multiple Potentiometers havinga common actuating member.

Another object of this invention of primary importance is to provide aconstruction that is time saving with respect to the task ofestablishing accurate phase relationships in the course of assembly. Thewiper on the rotor must be correctly phased relative to the resistor.The taps and terminals must be related with equal accuracy to theresistor.

Another object of this invention is to provide an improved, standardizedstructure that is simple, inexpensive and compact for joining togetherin a single housing assembly a plurality of electrical units adapted foroperation in common, and to provide new and improved methods of mountingand adjusting potentiometer elements.

Another object of this invention is to provide improved standardizedend-plate or partition structure for both joining together a pluralityof easing units and for positioning and supporting electrical apparatushoused therein, and to provide a simple fastening means for retainingtogether in ganged relation a plurality of pote'ntiometers or adjustableresistors.

Another object of this invention is to provide a potentiometer in whichthe taps thereon may be adjusted after the potentiometer is assembled toobtain a precise value between such taps.

Another object of this invention is to eliminate the need for trimmerresistors for correcting the electrical value, and to provide moreaccurate linearity by the elimination of said trimmer resistors.

A further object of this invention is to avoid the necessity of externalor additional parts, or the installation thereof, and to provide apotentiometer having the improved characteristics above noted withoutrequiring more than a resistance element, the required adjustable springtaps, a traveling contact, and the proper housing.

As will be explained, one of the features of the invention with regardto the foregoing is the concept of combining a slip ring on the rotorwith a wiper member that may be mounted on the rotor and staked inapproximate position simply by a metal stake through the rotor. When somounted the wiper member may be phase adjusted by simply rotating twotap terminals which will change the resistor position relative to thewiper member.

A further feature of the invention with respect to phase adjustment, andespecially the saving of assembly costs in making phase adjustments, isthe concept of making the various units of a ganged potentiometer freelyadjustable relative to each other.

A further feature of the invention with respect to simplicity ofstructure and ease of assembly is the concept of employing a resilientbrush structure for yielding engagement with a circumferential groove ofthe abovementioned slip ring for the purpose of energizing the slip ringand the wiper member carried thereby. 1n the preferred practice of theinvention, the brush structure comprises one wire arm adapted forengagement with the slip ring groove. The brush structure is resilientradially so that it may be freely assembled after the rest of thepotentiometer has been assembled as a unit, by simply pushing it throughits particular hole in the case wall as it does not matter which side ofthe slip ring the resilient brush contacts.

The brush structure is then brought into register for permanentcooperation with the slip ring. Such resiliency is a safeguard againstfailure of the brush structure under severe service conditions.

The above and other objects and advantages of the invention will beapparent in the following detailed description of the presentlypreferred practices of the invention, considered with the accompanyingdrawings, which are to be regarded as merely illustrative.

In the drawings:

Fig. 1 is a longitudinal sectional view of one form of the invention,the rotor assembly being shown in side elevation;

Fig. 2 is a transverse section taken as indicated by the line 22 of Fig.1;

Fig. 3 is a transverse section taken as indicated by the line33 'of Fig.'1.

Fig. -4 is a sectional view of a potentiometer embodying the invention,with a wiper element at forty-five degrees rotation in relation to thealignment holes.

Fig. 5 is a sectional view of the potentiometer embodying the invention,with the wiper element at ninety degrees rotation in relation to thealignment holes.

.Fig. 6 is a sectional view of apotentiometer embodying the invention,with one tap.

Fig. 7 is a sectional viewof a potentiometer embodying the invention,employing two taps.

Fig. 8 is a sectional view of a potentiometer embodying the invention,employing three taps.

Fig. 9 is a sectional view of a potentiometer embodying the invention,employing five taps.

Figs. 10-17 show schematically a few of the circuits i which thisinvention may be applied.

Referring now to Fig. l, in accordance with this invention in a specificembodiment thereof, a plurality of potentiometers 46 are mounted incasingiunits or cylindrical walls I39-41 having standardized rings 37and shoulders 38 which mate with adjoining units and which also serve.to position and support potentiometer elements and associated electricequipment within the respective .casing units. The synchro-mount endcover 7 is connected to the cylindrical "Wall 39, and carries a suitablebearing 11 for one end of a rotor shaft 11. At the other end of theassembly, a cap 3 or end cover is connected with the end cylindricalwall 41 to complete the composite casing andto carry a second bearing 2for the other end of the rotor shaft.

The potentiometer units 445, and the synchro-mount end cover 7, and theend cover 3, may be held together in assembled relationship in anysuitable manner. In the preferred practice of the invention, however,two .screws r 10 are employed for this purpose and the holes throughwhich they pass, 34 in Fig. 2, maybe used to relatively check and alignthe potentiometer units electrically to each other in an angularrelation previous to their assemblyin a ganged relationship.

Each of the'cylindrical walls 39-41 is formed with an internalcircumferential shoulder 18 against which is positioned a suitableresistor, having a configuration of a split ring, this resistor beingresistor v of Fig. 1. In a preferred practice of the invention, eachresistor 15 comprises a ring-shaped core 12 on which is wound a helicalresistance wire. The core 12 may be made of metal coated with suitableinsulating material toisolate the resistance wire.

The rotor shaft 1 is part of a rotor assembly which includes a sleeve 42of suitable non-conducting material as shown in Fig. 2, which is slippedonto the rotor shaft. Slip rings 22, which are inthe form of coin silversleeves, are assembled over this plastic non-conducting sleeve 42 whichinsulates them from the rotor shaft, there being one slip ring 22 foreach of the potentiometer units.

Each of the slip rings 22 is suitably adapted to maintain constantcontact with the corresponding brush means 19, and contacting each slipring is a suitable resistor con- 1 tact or wiper element 26 with acoined tip 27 for contact with the corresponding resistor 15.

The space between the ends of core 12 in Fig. 3 is filled with asuitable material 13 forming a bridge for the Wiper.

The wipers and the slip ring of one unit are insulated from those of andadjacent unit by suitable insulators 23 which also provide insulationfor the metal washers 25. The rotor assembly is then staked into a rigidassembly by tapered pin 20 driven into a tapered hole 21, whichcompletes the rotor assembly.

In the preferred construction shown in the drawings, each slip ring 22is provided with a shallow circumferential groove 24 for cooperationwith the corresponding brush means 19. It is contemplated that the brushmeans 19 will be of resilient construction for the purpose ofmaintaining pressure contact with the slip ring in the groove 24 in thenormal operation of the potentiometer. In the construction shown, eachbrush means 19 comprises a resilient wire arm, a portion of which isformed into a .helicalspring.

Each of the potentiometer units has two external terminals, in Fig. 2designated 45 and 46 to energize the resistor 15 and has a terminal'8for connection to the corresponding wiper arm .26. Terminal 8 carries asuitable brush means designated 19. The terminals have hexagon shanks 9by which their adjustments are made.

In order to provide an accurate and precise termination for theunconnected ends of the resistance unit 15, variable or adjustablespring taps 43 and 44 in Fig. .2, are provided which permit theeffective length of the resistance wire 15 to be accurately adjustedwithin very close limits. Engagement between the spring taps 43 and 44on the casing 41 and the resistance wire 15 on the mandrel 12 is assuredby the wire for each tap having a high tensile strength and springingagainst the potentiometer wires, so that the taps thereon firmly engagethe wires on the resistance member. Terminal .slots 14 allow clearancefor adjusting the taps. A suitable tool may be engaged with the feedthroughs 9 in Fig. 2, so that either a clockwise or counter-clockwiserotation imparted to the tool will .cause the-.spring tap 43 or 44 toslide along the resistor and .either increase or decrease the resistanceby altering the position of the termination.

It will be observed that the end termination of the resistance wire maybe precisely adjusted to provide the desired resistance value, orangular resistance length.

The potentiometer may be provided with adjustable taps at its end andwith additional taps between its ends to provide precise and accuratevalues of resistance, as will hereinafter be described in detail.

It is not necessary to use clamping means of special construction tointerconnect the units. The units set to approximate alignment bysuitable instruments are simply assembled with the two screws 10 throughan alignment "hole or holes 34.

By means of measuring instruments, the positions of the resistance unitsin their individual casings may be adjusted relative to each other 'soas to provide the desired synchronization or adjustment of one resistorrelative to the other. This adjustment is accomplished by rotating thefeed throughs to the adjustable spring taps until the desired adjustmenthas been obtained. A simi larprocedure is followed for successivecasings. By this arrangement the desired accuracy in the control may beobtained.

Using the adjustable taps as described herein, it is not difficult toterminate the resistor at accurately predetermined points. Theadjustment of the potentiometer to fit a particular instrumentserrorsiso'as to render the correct signal output is therefore an easytask. The terminal feed through for each corresponding tap being of aclose fit requires no additional tightening after it is once set inposition. Although these feed throughs may be placed in position verysimply with the proper tools, the high friction from their close fitwill not allow them to be moved out of this position through accident.In the preferred embodiment the cylindrical walls 39 to 41 are made oflinen base melamine which serves as an insulator as well as the resistormount. The linen fibers aid in holding the feed throughs in their setpositions. The cylindrical casings may, when required, be made ofaluminum into which a melamine sleeve may be pressed for the feedthrough to pass through thus insulating it and giving it the desiredfrictional contact.

A problem is to afford a smooth wiper action throughout the rotary rangeof the potentiometer with absolutely constant voltage provided betweenaccurately predetermined points of the resistor withoutthe generation ofspurious signals or other troublesome phenomena.

Various attempts have been made to solve the problem by having the wipertraverse the resistor through the whole range of potentiometer operationwith the successive turns of the resistor electrically interconnectedover the portion of the range in which constant voltage is desired. Suchattempts do succeed in providing a smooth wiperaction, thus eliminatingany spurious signals that are due solely to the wiper action.Unfortunately, however, other difiiculties are introduced.

One expedient, for example, is to place a conductor .such as a thin wirealong the core of the resistor in electrical contact with the successiveturns of the resistance wire. It is exceedingly difficult to fabricatesuch a resistor with accurate placement of the transition points,especially so since the helically wound resistance wire tends to pressthe ends of the inserted conductor into the underlying core material.

Another expedient is to deposit a layer of metal along the resistor, thedeposited metal interconnecting the successive turns of the resistorover the desired range of constant voltage. It has been found, however,that, notwithstanding great care and skill, such a deposit incombination with the turns does not provide uniform voltage.Consequently the traveling wiper in traversing the shorted portion ofthe resistor creates an undulating signal. which is imposed on a controlsystem.

Broadly described, the present invention meets this problem by ashorting connection between one of the terminals and one of the taps, orif a portion of the resistance along the coil is to be shorted out soasto provide a portion of constant voltage, this may be accomplished byplacing a shorting connection between two of the taps. The terminals andtaps could then be adjusted to the length of that portion of the rangeof potentiometer operation over which constant voltage is der sired.Thus the area of equal voltage that the wiper may traverse can be variedto a very accurate length by the use ofthis invention.

This invention contemplates providing for compensating-apotentiometer-the compensation being applied in such a manner as toprovide not only the exact total resistance of a potentiometer required,but also to provide the exact function of voltage output vs. shaftdisplacement requiredfwith a given fixed input voltage and a given loadresistance connected to the wiper of the potentiometer.

If it is required that apotentiometer, having applied to one of itsfixed terminals a given fixed voltage, must supply as the output of itswiper a voltage which is a given function of the potentiometer shaftrotation, it can be seen that the simple utilization of a potentiometerwhose resistance vs. shaft rotation function required would not sufiicebecause of the presence of the load resistor. This insufiicientlyfollows because though the potentiometer is connected as a voltagedivider, the voltage at the Wiper may not be computed simply by sealingdown the supply voltage by the ratio of the resistance between the wiperand one fixed terminal to the total resistance of the potentiometer, butmust be computed by reference to Kirchofis law of currents since ofresistance wire as a resistance element.

the wiper does draw current. In addition, while potentiometers haveheretobefore been available which would provide a given function ofresistance vs. shaft rotation within limits of a few percent, since thetolerance could not be entirely eliminated, it has been necessary toadjust the resistance vs. shaft rotation characteristics of eachpotentiometer before it could be inserted in a circuit to attain thedesired precise result. In addition, tolerances exist upon the totalresistance of a potentiometer which therefore necessitated theadjustment of the total resistance of a potentiometer before it could beutilized in a precision computer circuit.

In resistance devices in which a resistance wire is wound in a pluralityof turns on a support, it is sometimes found necessary in order toobtain predetermined precise resistance values to tap the resistancewireat precise points instead of simply connecting ends of the resistancewire to terminals and using the whole length The point of tapping mayhave to be so precise that a particular turn of the resistance wire mustbe tapped.

It will be appreciated, therefore, in constructing potentiometers of theself-balancing type that it is highly desirable to construct theresistance unit in such a manner *that a precise over-all resistancevalue is assured and that some means should be provided for adjustingthe value or effective length of the resistance unit after it has beenassembled between terminations thereon.

The ideal resistance unit for a potentiometer of the self-balancing typeshould consist of a bare wire of resistance material so disposed thatthe sliding contact or arm may engage any point along its length andthat the length thereof would be precise and of an exact value. However,due to the physical difficulties of obtaining suitable resistance valueswithin the available dimensional limits, it is seldom that this idealcan be obtained without resorting to complicated mechanisms which, inthemselves, tend to introduce errors that offset the precision attendanton the use of the continuous bare wires.

In potentiometers of the self-balancing type, it is usual, therefore,for the slide wire or resistance unit to be in the form of a continuouswinding on a card or mandrel, with the turns of wire along one edgethereof exposed for contact with a sliding arm or contact member atsuccessive turns. By the use of a very fine wire and a great many turns,the potential drop between successive convolutions may be made so smallthat a single step, that it, from one turn to the other or a momentaryshortcircuit between turns is not perceptible in its effect on theapparatus in the circuit of which the potentiometer forms a part. But inthe use of a very fine wire to obtain a great many turns the resistancewould be increased to a quantity that is too high to be used in somecircuits. This invention provides a means whereby a high resistancepotentiometer may be reduced to a total relatively low resistance, andit may fit into a low resistance circuit and still maintain theadvantage of a great number of turns.

Fig. 4 to 9 illustrate the relative positions of taps on gangedpotentiometer uni-ts in accordance with circuits illustrated in Figs. 10through 17. The application of this invention in various circuits areinnumerable, and these circuits are given only to illustrate theversatility of this invention.

Fig. 10 shows the potentiometers in Figs. 4 and 5 ganged together, thedistance A being the distance to the electrical center of resistor 54from terminal 55 and the distance C being the distance from theelectrical center of resistor 50 from terminal 53. Therefore thedifferences between the electrical centers in this ganged relationshipis the distance B.

Fig. 11 shows a means of very accurately center tapping resistor 59.This is obtained by a combination of the potentiometer in Fig. 5 and theresistor unit of Fig. 6. A voltage is placed across resistor 59 andresistor 54, and tap 58 is adjusted for as near electrical center of I?resistor 59 as possible. Contact 61 is .not used in this combination;therefore, as Wiper '56 moves along resistor 54, a very fine adjustmentof the electrical center of resistor '59 is obtained.

Fig. '12 shows resistor 64 of Fig. 7 in a circuit to render a portion ofan end A at zero voltage and a portion of an end C at maximum voltage bythe use of shorting c011- nections 69. The lengths A, B, and C may beadjusted very accurately.

Fig. 13 shows the resistor '59 from Fig. 6 with the center tap 58 placedfar to one side. Two terminals 6-; and 58 may be adjusted to accuratelycoincide with the zero percent and 106 percent of mechanical movement.The value of resistance for distance A is very high, while the value ofresistance for distance B is near the amount required for the givencircuit. Terminal 62 may be adjusted to give the exact requiredresistance for the given circuit as the sections A and B are parallelwith each other. The wiper 61 then travels between terminals 58 and 60rendering very fine divisions of the voltage therein.

Fig. 14 shows resistor 72 of Fig. 8 in a circuit to render a non-linearvoltage output in relation to the wiper movement. The load resistor 87impresses voltages on resistor 72 differing from the percent ofmechanical movement. curately.

Fig. 15 shows the resistor 79 of Fig. 9 and resistor of Fig. 4 ganged togive a varied function output signal. The wipers 82 and 52 are on acommon shaft. As the wipers move along their respective resistors 79 and5t), wiper 52 produces a linear voltage while wiper 82 produces acycling voltage from zero to 100 percent and back to zero returning to100 percent etc.

Fig. 16 shows a complex circuit using the resistors of Fig. 7 and Fig.9. The wipers 66 and 82 are assembled on a single rotor and the twopotentiometer units are in a ganged relationship. Now consider only thevoltage output of wiper 32 from resistor 79. As the wiper 82 traversesfrom terminal 81 to terminal 33 there is a rapid decrease in voltageuntil terminal is reached. The voltage then remains constant untilterminal 78 is reached and again a rapid decrease in voltage to zero asterminal 77 is reached. Then there is a rapid increase in voltage untiltap is reached. Then there is a slower increase in voltage until tap 84is reached and then a rapid increase in voltage to percent as terminal33 is reached. Simultaneously wiper 66 traveling from terminal 65 has arapid decrease in voltage until terminal 63 is reached. Then a very slowincrease in voltage until terminal 67 is reached. This arrangement oftaps and terminals provides the means by which the desired voltagefunction can These differences may be adjusted very ac be set withextreme precision from data presenting the function in a tabular form,by shifting the taps along the resistor until the desired variations areobtained in a very accurate relationship.

Fig. 17 shows the potentiometer as described in Fig. 7 with the resistor64 Wound of very fine wire and a great many turns. In normal practicethis winding arrangement would increase the resistance to a value toolarge to be utilized some circuits. But to maintain the fine resolutionfeature of this arrangement, the portions of resistor 64 represented byA and C are placed in parallel with that portion B thus giving a totaldecrease in resistance. Therefore, as wiper 66 moves between terminal 63and 68 along resistor 64 we obtain a very fine gradient of voltageincrease. Taps 63 and 63 may be set for very accurate mechanicallocations equal to the mechanical movement of wiper 66 while terminals67 and 65 may be adjusted to render a very accurate total resistance.

Cir

While we have elected to describe our invention as embodied 'in 'a"rotary type potentiometer, it is obvious that the same principles areapplicable to helical potentiometers as well as potentiometers thatoperate with straight line relative motion. It is also apparent that ourspecific disclosure of'the presently preferred practices of theinvention will suggest to those skilled in the art various changes,substitutions and other departures that properly lie within the spiritand scope of our appended claims.

We claim:

1. In a variable resistance device comprising a mandrel of insulatingmaterial and a resistance wire wound in a plurality of convolutions onsaid mandrel, the combination comprising: a casing; means in said casingfor supporting said mandrel; an adjustable Wiper in engagement with saidresistance wire; at least one .adjustable terminal including a feedthrough passing through said casing and provided with means by which itmay be rotated clockwise or counter clockwise; a spring tap connected tosaid feed through for making pressure contact with the resistance wire,said spring tap being adapted to be moved along said resistance wireupon rotation of said feed through to vary the point of contact of saidtap with said resistance wire.

2. A variable resistance device according to claim 1, including anadditional adjustable terminal similar to said one adjustable terminalfor tapping said resistance wire between its terminals.

3. In .a rotary variable resistance device, the combination of: acasing; afixed curved resistor secured within said casing; a rotary slipring having a circumferential groove; a wiper associated with said slipring to electrically connect the slip ring with said resistor; a brushstructure in contact with said slip ring, said brush structure having aresilient 'arm on a feed through passing through said casing tomakecontact withsaid groove, said resilient arm being smaller in diameterthan said feed through, whereby said brush structure may be insertedthrough said casing after said resistance device has been assembled.

4. A rotary variable resistance device according to claim 3, in whichpart of said resilient arm is formed like a helical spring, smaller indiameter than the feed through, over a portion of its length to increaseits resiliency.

5. In a rotary variable resistance, the combination of: a curvedresistor; a wiper in contact with said resistor; actuating means tocause relative rotation between said resistor and wiper about a givenaxis; means for adjustably gauging together a plurality of suchresistors in adjusted angular relation to each other, comprising acasing for each resistor, each casing having adjustable terminals andalignment holes to which the electrical positions of .the resistorshavebeen adjusted; and alignment screws to pass through these alignmentholes to lock the resistors relative to each other.

6. A rotary variable resistance device according to claim 5, including acommon control shaft for said resisters.

References Cited in the file of this patent UNITED STATES PATENTS1,026,798 Gill May 21, 1912 2,592,392 Canziani et a1 Apr. 8, 19522,657,295 Barclay Oct. 27, 1953 2,662,146 Wilentchik Dec. 8, 1953

